1. Field of Invention
This invention relates to operating fluid ejection heads.
2. Description of Related Art
Printers generally print images on a recording medium such as paper. A popular type of printer, the ink jet printer, expels ink droplets from a printhead, and propels the ejected ink droplets toward the recording medium or an intermediate surface, such as a coated receiver drum. One method of enhancing productivity in the ink jet printer is to use printheads that span the full width of the print process. One embodiment of such a printhead is the full width array printbar, in which all drop ejectors of a given color, or of all four primary colors (CMYK), are arrayed on a single marking bar or element. An alternate embodiment is the staggered full width array, which is comprised of partial width sections of drop ejectors, stitched together in a staggered array to span the width of the print process. Both embodiments have a large number of ejectors that are spaced apart and orientated towards the recording medium. Ink droplets are ejected from the ejectors of the array as the recording medium or intermediate surface moves past the array. The ink droplets make contact with the recording medium to form dots that create images on the recording medium or intermediate receiving surface.
Full width array printbars, such as that shown in FIG. 4, are used to provide enhanced productivity by reducing the number of passes of the receiving medium under the printing array. The highest productivity is obtained with single pass printing. As shown in FIG. 4, the full width printing array is fixed, and the recording medium moves past the array. A set of ejectors ejects ink droplets at particular positions across the recording medium. The set of ejectors may produce ink droplets of colors, for example, using CMYK color values, where C represents cyan, M represents magenta, Y represents yellow and K represents black. FIG. 4 shows individual print bars for each color. A common embodiment (not shown) is to include all colors (CMYK) on a single bar, thus avoiding color-to-color registration issues. For a full width array printbar having 600 ejectors per inch per color, a minimum of 5,100 total ejectors are required for each color for a recording medium that is 8.5 inches wide.
For various reasons, the ejectors in a full width array are subject to failure. The failures may be classified in 3 major categories. A given drop ejector may fail such that no ink is ejected, resulting in a missing jet. Off-axis jetting, in which drop position on the receiving medium is affected, is a second major failure mode. Finally, ejectors may fail to eject an adequate amount of ink, resulting in weak jetting. Due to the printing architecture used with full width arrays, particularly with single pass systems, the failures above result in streak artifacts in the direction of the print process (perpendicular to the printing array). This occurs because the position of the failed ejectors relative to one dimension of the recording medium does not change. That is, the fixed, full width array printbar extends across the recording medium and the recording medium moves past the full width array printbars in a direction perpendicular to the linear array of ejectors, but does not move parallel to the full width array printbars. Failed drop ejectors can often be recovered by a printhead maintenance action, which comprises cleaning the front face of the full width array and purging to remove air bubbles trapped in the ejectors. When the failure cannot be recovered by such a print head maintenance action, it is termed a chronic failure, which can only be remedied by replacing the entire array or array section. Because human visual system is very sensitive to structured objects such as lines, for high quality printing, a full width array print bar with one or more failed drop ejectors must be replaced, which results in expensive repair cost as well as printer down time.
The invention provides systems and methods that can reduce artifacts caused by failed or malfunctioning ejectors without introducing other artifacts.
This invention separately provides systems and methods that do not require replacement of a full width array print bar with one or more failed or malfunctioning ejectors.
Selective replacement methods and systems according to this invention, use a redundancy inherent in the full width array printing system to compensate for one or more non-firing or malfunctioning nozzles in one or more colors. As an example, absorption spectra of the four colors cyan C, magenta M, yellow Y and black K usually overlap to some extent, thus there is redundancy that can be used to hide the artifacts, such as streaks. Since the human visual system is much less sensitive to chrominance changes than to luminance changes, matching the luminescence of a pixel where a color is missing, using a random or pseudo-random process, can adequately remove such a streak without introducing other objectionable artifacts.
In various exemplary embodiments, methods and systems for selectively printing a pixel at a print location having a missing color include determining if the missing color is to be printed for that pixel, determining for a pixel that is to include the missing color, which colors are to be printed based on a color value to be printed for that pixel, and selecting, when the missing color is to be printed, at least one color to be printed in place of the missing color based on at least a pseudo-random process. In various exemplary embodiments, the pseudo-random process is a stochastic process.
In various exemplary embodiments, methods and systems include determining a color value of at least one neighboring pixel, wherein selecting at least one color to be printed in place of the missing color includes selecting the at least one color based on the at least one determined color value of the at least one neighboring pixel. In various exemplary embodiments, methods and systems include replacing a color value of at least one neighboring pixel with another color value that corresponds with the at least one color to be printed in place of the missing color.
In various exemplary embodiments, methods and systems include determining a type of image to be printed, wherein selecting the at least one color to be printed in place of the missing color includes selecting the at least one color based on the determined type of image to be printed.
In various exemplary embodiments, methods and systems include not printing a color in the pixel with the missing color based on the pseudo-random process.
In various exemplary embodiments, methods and systems include determining if a given print location has a missing color. In various exemplary embodiments, methods and systems include determining if at least one ejector is malfunctioning or has failed, and disabling the at least one ejector that is malfunctioning or has failed from ejecting ink droplets.